Power amplification devices configured to amplify high-frequency signals have two conflicting requirements, i.e., high efficiency and low distortion. Switching amplifiers, such as class D, E and F amplifiers, in theory have 100% efficiency. However, in order to superimpose amplified information on output signals, the power supply voltage needs to be varied by modulating the voltage, or the on/off timing or ratio of switching needs to be varied.
Recently, various approaches have been proposed that attempt to have both high efficiency and low distortion at high frequencies, and a promising approach is to keep the power supply voltage of a switching amplifier constant. For example, the approach of U.S. Pat. No. 7,352,237 (Reference 1) compensates for distortion of a high-frequency and high-efficiency class E or F switching amplifier with a negative feedback, and generates a pulse input signal of the switching amplifier in a sequencer configured to turn off the pulse input when the voltage of a high-frequency error signal turns positive from negative, and return the pulse input signal to an on-state after a predetermined period of time has elapsed. However, this approach poses a problem that efficiency is severely degraded at high frequencies, since the on/off timing and ratio in a backoff state greatly deviates from those at the maximum output power optimized for the output tuning circuit and generate a large amount of out-of-band noise.
In order to solve this problem, the approach of U.S. Pat. No. 7,474,149 (Reference 2) varies the power voltage of a switching amplifier according to the envelope curve of a high-frequency input signal. This approach generates less out-of-band noise and provides a higher efficiency switching amplifier than Reference 1, since the on/off timing and ratio approximate those of the state in which the output tuning circuit is optimized.
The approaches of References 1 and 2, however, pose a problem that synchronization cannot be established with the resonance phenomenon of an output tuning circuit of a switching amplifier. This degrades efficiency at high frequencies and quantized noise remains as out-of-band noise. Accordingly, a band pass filter needs to be provided on a high-power output side so as to prevent out-of-band noise components.